A semiconductor injection laser typically comprises a body of semiconductor material having a thin active layer between cladding regions of opposite conductivity type. To increase the output power, a guide layer having a refractive index which is intermediate between that of the active and cladding layers may be interposed between one of the cladding regions and the active region. Light generated in the active layer propagates in both the active and guide layers thereby forming a larger beam at the emitting facet of the body. The cavity region comprising an active layer or the combination of a guide layer and an active layer restricts oscillation in the transverse direction, the direction perpendicular to the plane of the layers, to the fundamental optical mode. In the lateral direction, the direction in the plane of the layers and perpendicular to the axis of light propagation, it has been found useful to introduce structural variations which produce an optical waveguide which in turn restricts the oscillation to the fundamental optical mode. Lasers incorporating transverse and lateral waveguides, such as that disclosed by Botez in U.S. Pat. No. 4,347,486, incorporated herein by reference, may have output powers in excess of about 40 milliwatts in the fundamental lateral and transverse mode.
To increase the output power in the laser beam beyond the capability of such an individual laser, monolithic arrays of spaced-apart laser devices, such as that disclosed by Botez et al. in U.S. Pat. No. 4,547,396, incorporated herein by reference, have been fabricated wherein the modes of oscillation of the individual lasers are coupled to one another to form a single phase-locked coupled oscillator. Such arrays that operate in a single longitudinal mode to output powers as high as 80 milliwatts have been observed. However, a problem with the typical phase-locked array is that nearly all of the devices have operated in such a manner that the far-field radiation pattern in the lateral direction has consisted of two lobes symmetrically located about the normal to the emitting surface of the array. This far-field distribution results because of gain considerations that cause adjacent devices to operate such that their relative phase is 180.degree.. This far-field pattern is undesirable from a systems viewpoint because it requires excessively large optics to collimate the emitted beam. It would be desirable to have a phase-locked semiconductor laser array operating in the single transverse and lateral mode and having a far-field radiation pattern comprising a single lobe oriented perpendicular to the emitting surface of the laser.